For each walk, the starting genotype

was randomly drawn to have relatively low fitness (see for details).

**A.** Intervals covering fitnesses between the 2.5 and the 97.5 percentiles are shown for the first (ab), second (

), and third (

) genotypes in randomly generated adaptive walks, with dots indicating the medians. The genotype

is the remaining genotype in the quadruple as shown in . The blue “Control” interval corresponds to randomly selected genotypes. The skew visible in the ab interval is due to the fact that the initial genotype of a fitness walk is drawn from a lower tail distribution.

**B.** Intervals for the fourth, fifth, and sixth genotypes in randomly generated adaptive walks. The increased fitness of the aB genotypes in B relative to that of A is due to the fact that

, and thus there is some correlation between neighboring genotypes. In both diagrams, the dependency of sign epistasis on regression to the mean is apparent.

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